This invention generally relates to an improved method of electrodepositing a water-soluble or water-dispersible coating resin onto a conductive surface, and, in particular, is directed to the formation of a chromium phosphate type conversion coating prior to electrocoating to provide improved cured paint adhesion.
The electrodeposition of water-based coatings, commonly termed electrocoating, is a widely used process which has many advantages over other methods of coating, such as spraying, dipping, rolling and the like. The advantages of electrocoating are well known. The process deposits a film of uniform thickness on essentially any conductive surface, even those which have sharp points and edges. The electrocoated film when applied is relatively water-free, and, thus, will not run or drip when taken out of the bath. Because little or no organic solvents are used in the resin system, the process is essentially fumeless and requires no extensive fume collection and incineration equipment. This latter point is important in view of the increased concern over environmental pollution. An additional advantage is the fact that a second or top coat can be applied over the electrocoated film without curing the electrocoated film and then both coats can be cured in one baking operation. By eliminating the necessity of two furnaces, the cost of a two-coat process can be considerably reduced.
The electrocoating process generally comprises immersing the article to be coated into the electrocoating bath, usually as an anode, and passing a current through the bath between the article and electrode. The process usually is self-arresting in that as the thickness of the coating increases, the resistance thereof also increases, thereby limiting the amount of coating which is electrodeposited.
The overall anodic electrocoating process involves four separate processes, namely, electrophoresis, electrocoagulation, electroendosmosis and electrolysis. Electrophoresis involves driving negatively charged resin particles to the positively charged anode which is the article to be coated. In electrocoagulation, the resin particle loses a negative charge in the close vicinity of the anode or in contact therewith which causes the resin particles to lose their stability and coagulate on or about the anode. Electroendosmosis occurs during and after electrocoagulation and involves driving water out of the coagulated resin, thus, in effect, drying out the electrodeposited coating. Electrolysis also occurs causing evolution of hydrogen at the cathode and oxygen at the anode. With aluminum and other reactive metals, anodic oxidation usually occurs at least initially. Most commercial electrocoating systems are anodic in that the article to be coated is the anode in the electrocoating cell as described above. However, in certain situations, cathodic deposition, wherein the article is the cathode and the coating resin carries a positive charge, has been found useful.
Most commercially available resins for anodic electrocoating generally are polycarboxylic acid-based resins and frequently are acrylic or methacrylic acid-based resins. To solubilize the resins, they are usually completely or nearly completely neutralized by a base, such as an amine or KOH. With cathodic electrocoating, the resin generally is a basic polymer resin which has been neutralized with a soluble acid. During anodic electrocoating, the amine takes on a hydrogen ion and is driven to the cathode where H.sub.2 is liberated. The amine or other neutralizing agent is not deposited in the coating and will stay in the bath except for small amounts which are lost through dragout. To maintain a relatively constant level of amine, it is preferred to treat the bath in an ultrafilter or other suitable device to remove amines and other low molecular weight contaminants from the bath. For an excellent discussion on the use of ultrafilters in purifying electrocoating baths, see the article "Ultrafiltration of Electrocoating Systems", in Nonpolluting Coatings and Coating Processes, Plenum Press (1973) edited by J. L. Gardon and J. W. Prane. Coupling agents which assist in solubilizing the paint resin are frequently added. The resin can be pigmented or clear as desired.
Due to the fact that after painting most coil coated aluminum sheet is fabricated into such products as roofing, siding, facia systems, gutters, downspouts, shingles and the like, the adhesion between the paint and the aluminum substrate must be at sufficiently high levels to accept the deformation attendant with such fabrication without cracking or pulling away from the aluminum substrate. Much of the prepainted stock is embossed with a pattern (e.g., wood grain and the like) prior to fabrication which provides additional stresses on the conversion coating and paint layers.
Conventional coil coating practice has been to first clean the sheet, form a conversion coating thereon, and then roll one or more coats of paint. Most prepainted aluminum sheet has two coats, a primer coat and a top coat. The purpose of the conversion coat is to provide an improved base for the application of the coating resin so as to more firmly adhere the cured resin to the metal substrate and further to provide improved corrosion resistance.
The ferricyanide accelerated chromic chromate type conversion coating has been almost exclusively used in the continuous coil coating of prepainted aluminum sheet, particularly in those lines wherein the paint is applied by roll coating. This type of coating provides better corrosion resistance and paint adhesion with roll coated products than other conversion coatings, such as chromium phosphate type coatings. The aforesaid conventional coating practice has provided high quality aluminum products which have had service lives of 20 years or more under exposed outdoor condition.
Other conversion coatings, such as chromium phosphate coatings, have been tried in the past but generally they do not provide the level of corrosion resistance or paint adhesion provided by the chromic chromate coating. Chromium phosphate coatings are frequently used as a lacquer base for containers for food and beverages. Occasionally, such coatings are used on paint electrodeposition lines where discrete articles are coated; however, in these cases, there is less concern about adhesion because there is no significant deformation of the substrate and usually only one coat of paint is applied.
The ferricyanide accelerated conversion coating is believed to be CrFe(CN).sub.6.Cr(OH).sub.3.H.sub.2 CrO.sub.4 .4Al.sub.2 O.sub.3 .8H.sub.2 O. The chromium phosphate coating is believed to be Al.sub.2 O.sub.3 .2CrPO.sub.4 .8H.sub.2 O. The water content of the coatings can vary depending upon age and thermal treatment.
The bond between the paint and the substrate of prepainted aluminum sheet is usually evaluated by determining the minimum bend radius the sheet can withstand with no cracking and no removal of the paint coating from the bend area by cellophane tape (e.g., No. 600 Scotch Brand). For commercial acceptability, the prepainted sheet must have a minimum bend radius of 2T (T = thickness of the sheet). Commercially prepared prepainted strip usually has a minimum bend radius between 1T and 2T depending upon temper. If otherwise, the prepainted sheet is incapable of withstanding the deformation characteristic in the fabrication of roofing, siding and similar products.
However, in evaluating the replacement of the primer roll coating step of prior practices with a primer electrocoating step, it was found that such coated products could not be fabricated into products, such as roofing, siding, gutters and the like, because the adhesion between the cured coating and the aluminum substrate was so low (minimum bend radius greater than 2T) that the coatings cracked and pulled away from the aluminum substrate during such fabricating procedures.
It is against this background that the present invention was developed.